This invention relates to medical devices, such as vascular filters to be used in a body lumen, such as a blood vessel, with improved strength and flexibility. A filter according to the invention includes a proximal frame section, a distal section and a flexible thin membrane with perfusion holes of a diameter that allows blood to pass, but prevents the movement of emboli downstream.
Both sections can be collapsed into a small diameter delivery catheter and expanded upon release from this catheter. The membrane has a proximal entrance mouth, which can be expanded, or deployed, substantially to the same size as the body lumen. It is attached to the proximal frame section, which has the function to keep the mouth of the membrane open and prevent the passing of emboli between the body lumen wall and the edge of the filter mouth.
In order to have a good flexibility, the membrane is made extremely thin. Normally this would create the risk that the membrane could tear easily, which could cause problems because emboli and pieces of the membrane would then be carried downstream from the filter site.
U.S. Pat. No. 5,885,258 discloses a retrieval basket for catching small particles, made from a slotted tube preferably made of Nitinol, a titanium nickel shape memory alloy. The pattern of the slots allows expansion of the Nitinol basket and by shape setting (heat treatment in the desired unconstrained geometry) this basket is made expandable and collapsible by means of moving it out or into a surrounding delivery tube.
In principle, a distal filter is made of such an expandable frame that defines the shape and enables placement and removal, plus a filter membrane or mesh that does the actual filtering work.
Sometimes the expandable frame and the mesh are integrated and made from a single material, for example Nitinol, as disclosed in U.S. Pat. No. 6,383,205 or US Published Application No. 2002/0095173. These filters do not have a well-defined and constant size of the holes where the blood flows through, because of the relative movement of the filaments in the mesh. This is a disadvantage, because the size of emboli can be very critical, e.g. in procedures in the carotid arteries. Further the removal of such a filter, accompanied by a reduction of the diameter, may be critical because emboli can be squeezed through the mesh openings with their changing geometry.
A much better control of the particle size is achieved with a separate membrane or filter sheath, which has a well-defined hole pattern with for example holes of 100 microns, attached to a frame that takes care of the correct placement and removal of the filter.
WO 00/67668 discloses a Nitinol basket that forms the framework of the filter, and a separate polymer sheath is attached around this frame. At the proximal side, the sheath has large entrance ports for the blood and at the distal side a series of small holes filters out the emboli. This system, however, has some major disadvantages. First of all, the closed basket construction makes this filter frame rather rigid and therefore it is difficult to be used in tortuous arteries. At a curved part of an artery, it may even not fit well against the artery wall and will thus cause leakage along the outside of the filter.
Another disadvantage of such filters is there is a high risk of squeezing-out the caught debris upon removal, because the struts of the framework force the debris back in the proximal direction, while the volume of the basket frame decreases when the filter is collapsed. Further the construction makes it very difficult to reduce the profile upon placement of the filter. This is very critical, because these filters have to be advanced through critical areas in the artery, where angioplasty and/or stenting are necessary. Of course the catheter that holds this filter should be as small as possible then. In the just described filter miniaturization would be difficult because at a given cross section there is too much material. The metal frame is surrounded by polymer and in the center there is also a guide wire. During angioplasty and stenting, the movements of the guide wire will create further forces that influence the position and shape of the filter, which may cause problems with the proper sealing against the artery wall. This is also the case in strongly curved arteries.
In U.S. Pat. No. 6,348,062, a frame is placed proximal and a distal polymer filter membrane has the shape of a bag, attached to one or more frame loops, forming an entrance mouth for the distal filter bag. Here the bag is made of a very flexible polymer and the hole size is well defined. Upon removal, the frame is closed, thus closing the mouth of the bag and partly preventing the squeezing-out of debris. This is already better than for the full basket design, which was described above, where the storage capacity for debris of the collapsed basket is relatively small. The filter bag is attached to the frame at its proximal end and sometimes to a guide wire at its distal end. Attachment to the guide wire can be advantageous, because some pulling force may prevent bunching of the bag in the delivery catheter.
It may be clear that it is easier to pull a flexible folded bag through a small diameter hole, than to push it through. However, the deformation of the bag material should stay within certain limits.
If the filter is brought into a delivery sheath of small diameter, collapsing the frame and pulling the bag into the delivery sheath causes rather high forces on the connection sites of filter to frame and/or guide wire. While the metal parts of the frame slide easily through such a delivery sheath, the membrane material may have the tendency to stick and in the worst case it may even detach from the frame and tear upon placement or during use, because of too much friction, unlimited expansion, crack propagation etc.
The connection of the filter bag to the frame is rather rigid, because of the method of direct attachment. Additional flexibility, combined with a high strength attachment spot would also be advantageous.
Methods for making kink resistant reinforced catheters by embedding wire ribbons are described in PCT/US93/01310. There, a mandrel is coated with a thin layer of encapsulating material. Then, a means (e.g. a wire) for reinforcement is deposited around the encapsulating material and eventually a next layer of encapsulating material is coated over the previous layers, including the reinforcement means. Finally the mandrel is removed from the core of the catheter.
Materials for encapsulating are selected from the group consisting of polyesterurethane, polyetherurethane, aliphatic polyurethane, polyimide, polyetherimide, polycarbonate, polysiloxane, hydrophilic polyurethane, polyvinyls, latex and hydroxyethylmethacrylate.
Materials for the reinforcement wire are stainless steel, MP35, Nitinol, tungsten, platinum, Kevlar, nylon, polyester and acrylic. Kevlar is a Dupont product, made of long molecular highly oriented chains, produced from poly-paraphenylene terephalamide. It is well known for its high tensile strength and modulus of elasticity.
In U.S. application Ser. No. 09/537,461 the use of polyethylene with improved tensile properties is described. It is stated that high tenacity, high modulus yarns are used in medical implants and prosthetic devices. Properties and production methods for polyethylene yarns are disclosed.
U.S. Pat. No. 5,578,374 describes very low creep, ultra high modulus, low shrink, high tenacity polyolefin fibers having good strength retention at high temperatures, and methods to produce such fibers. In an example, the production of a poststretched braid, applied in particularly woven fabrics is described.
In US Published Application No. 2001/0034197, oriented fibers are used for reinforcing an endless belt, comprising a woven or non-woven fabric coated with a suitable polymer of a low hardness polyurethane membrane, in this case to make an endless belt for polishing silicon wafers. Examples are mentioned of suitable yarns like meta- or para-aramids such as KEVLAR, NOMEX OR TWARON; PBO or its derivatives; polyetherimide; polyimide; polyetherketone; PEEK; gel-spun UHMW polyethylene (such as DYNEEMA or SPECTRA); or polybenzimidazole; or other yarns commonly used in high-performance fabrics such as those for making aerospace parts. Mixtures or blends of any two or more yarns may be used, as may glass fibers (preferably sized), carbon or ceramic yarns including basalt or other rock fibers, or mixtures of such mineral fibers with synthetic polymer yarns. Any of the above yarns may be blended with organic yarns such as cotton.